1. Field of the Invention
The present invention relates to a transmission for a tractor, and more particularly to a transmission for a tractor in which a high speed/low speed transmission part and a forward/reverse transmission part are integrated with each other, making the structure thereof simple and compact.
2. Description of the Prior Art
Generally, a transmission for a tractor is formed in type of a series of transmission parts that are connected in order from an engine. That is, the transmission generally includes a forward/reverse transmission part that changes a direction of rotating power of the engine to a forward/reverse direction, a main transmission part that changes speed in four-stage at an output side of the forward/reverse transmission part, and a sub transmission part that changes speed in two to four-stage at an output side of the main transmission part. The sub transmission part generally consists of a high speed/low speed transmission unit that is capable of selecting one from two-stage speed of high speed and low speed, and a super low speed transmission unit that reduces the speed of the output from the high speed/low speed transmission unit once more. The transmission includes a power take-off (PTO) transmission part that draws power for driving an attachment installed on a rear side of the tractor.
In connection with such a transmission, Korean Patent Application Publication No. 10-2006-0092873 has disclosed a power shift type transmission in which the transmission device of the main transmission part is automated using a hydraulic clutch instead of a conventional synchromesh unit.
FIG. 7 shows the whole structure of the transmission disclosed in that publication. As shown in FIG. 7, the transmission includes a forward/reverse transmission part 5, 6, a first main transmission part 10 (hereinafter, referred to as ‘a main transmission part’), a second main transmission part 11 (hereinafter, referred to as ‘a high speed/low speed transmission part’), and a sub transmission part 12 (hereinafter, referred to as ‘a super low speed transmission part’), which are arranged in a row (straight) to obtain forward 16-stage speed and reverse 16-stage speed. In the forward/reverse transmission part 5, 6, the main transmission part 10, and the high speed/low speed transmission part 11, the hydraulic clutches 5 and 6, 21 to 24, and 26 and 27 are respectively employed.
In such a transmission, although the power shift type by the hydraulic clutches is introduced for convenience of transmission, all gears from the forward/reverse transmission part to the super low speed transmission part are arranged longitudinally in a single row, i.e., in forward and reverse directions, so that a problem is caused in that a length of the transmission is lengthened excessively. For example, as the length of the transmission is lengthened, a distance between a front wheel and a rear wheel becomes far away to problematically increase a minimum turning radius as well as power consumption of the engine due to increased weight of the transmission.